One of its great strengths was being extensible. It's pretty easy to bake some functionality into Tcl and provide a new shell with it. This is what expect (http://expect.sourceforge.net/) is, for instance.
Another early advantage of Tcl was Tk, which provided Tcl extended with GUI stuff. Tk was cross-platform and established very early on, so if you wanted to provide one GUI for Unix, Mac and Windows, it was a good choice. It was very ugly for a long time; I hear it has gotten better lately but I don't know that myself. Tk is still built-in to Python as Tkinter. This is another specialty shell provided by Tcl, wish ("window shell").
Tcl was supposed to be good for embedding. The original author was supposedly using it for robotics. I haven't seen much energy there; I think Tcl's lunch probably got eaten by Lua some time ago. Lua is smaller, more like a modern programming language and easier to embed, but Tcl is more like shell scripting, so I guess there might be style reasons why you might prefer Tcl despite Lua's advantages.
Tcl is easy to disparage, but there's a lot of misinformation out there. At one time, it really only had one data type (strings). Quotation is a big topic in Tcl, but like Lisp, you can supply your own control structures. Everything is a command and commands have a lot of freedom about how to interpret their arguments. It is rather Lispy, unlike Lua. Tcl does have modern data structures and it does pretty snazzy things behind the scenes to optimize access to them while making things still seem to be strings for the cases where that matters.
Tcl is still around in some surprising places. I wouldn't expect there being a lot of new development starting with Tcl but it's kind of a fun language to know about. I got the newest edition of the book and it's a neat book, but I confess I haven't found ideal use cases for it.
